The present invention relates to a process for preparing acrylonitrile-styrene copolymer. More particularly, it relates to a process for preparing acrylonitrile-styrene copolymer having an excellent color and mechanical property by continuous bulk or solution polymerization.
Acrylonitrile-styrene copolymer is widely used for many application as a plastic material having good transparency, chemical resistance and mechanical property.
It is desirable that such a copolymer as widely used for many applications should be colorless.
Acrylonitrile-styrene copolymer is generally prepared by bulk or solution polymerization in the continuous process. This preparation method is advantageous to prepare pure copolymer but presents problems in that the reaction solution is hard to mix uniformly with difficulty in controlling the reaction temperature when the reaction mixture has high viscosity. The viscosity of the reaction mixture increases with the conversion ratio.
Especially, the reaction mixture becomes more viscous at the next reactor with a plurality of reactors connected with one another in series.
For acrylonitrile-styrene copolymerization, the composition of polymers is determined by the composition of monomers participating in the polymerization reaction. and acrylonitrile and styrene monomer polymerize at different rate. Thus the monomer mixture initially fed into the reactor varies in composition throughout the reaction in the preparation of the copolymer, and correspondingly, the composition of polymers also changes.
For such reason, the acrylonitrile-styrene copolymer formed in the preparation process becomes a mixture that has the composition distribution in a range. If the reaction mixture is not well mixed or the reaction is not accomplished uniformly, the distribution of the molecular weight and composition of the copolymer becomes broader. Broad distribution of molecular weight and composition is undesirable because broad distribution of composition of acrylonitrile-styrene copolymer resin deteriorates the transparency, color and mechanical property of the shaped articles. It is thus importance that the distribution of composition and molecular weight of acrylonitrile-styrene copolymer should be narrow in order to obtain copolymer of high quality.
Another problem possibly encountered in the polymerization step of the preparation process is formation of gel-like polymer. The gel formed in the polymerization step is an insoluble substance containing much acrylonitrile and has a very low thermal stability, causing red and black spots on the shaped articles and deteriorating the external appearance of the articles.
Such a gel is formed due to non-uniform composition locally in the polymer mixture, especially with unbalanced composition caused by insufficient mixing of the polymerization solution and non-uniform reaction in the polymerization step. The gel is ready be formed in the apparatus for processing the vapor of monomer mixture.
The phenomena causing the prescribed problems are usually due to high viscous polymerization solution which is the feature of acrylonitrile-styrene bulk copolymerization.
Use of a solvent such as ethylbenzene or toluene in a large amount may solve the problem resulting from the use of a polymer solution of high viscosity but has limitations in that the yield of production is decreased and the load of devolatizer increases.
Many methods to maintain the conditions for reaction uniform have been suggested as follows in order to eliminate the problem caused by the characteristic of acrylonitrile-styrene copolymerization itself:
U.S. Pat. Nos. 4,524,196, 4,324,868 and 2,769,804 describe a method for maintaining polymerization uniform in the reactor by using a special catalyst, or recycling a part of polymer solution back to the reactor. U.S. Pat. No. 3,838,139 discloses a method for enhancing the efficiency of mixing the reaction solution with horizontally arranged cylinder-like mixers.
In addition, EU Patent No. 344,437 and U.S. Pat. No. 3,336,267 suggest the use of proper solvent to make the reaction solution uniform, and U.S. Pat. Nos. 4,328,327 and 4,141,934 and JP Patent No. 7-309905 describe a method for feeding the monomer in different composition into the respective reactors.
These conventional methods are effective to improve the problems partly but involve disadvantages such as use of very expensive equipment and complex preparation process.
Particularly, the method for feeding the monomer in different composition into the respective reactors is expected to compensate for the change of monomer composition with conversion ratio in the next step reactor, requiring a separate apparatus for varying the composition of monomers fed into the reactors with a consequence of an increase in the costs. This method is inapplicable to the actual preparation process because there exists a large difference in the composition of monomers between the liquid and gas phases in the reactor and the composition of monomers is hard to measure. Furthermore, for a polymerization of high level of acrylonitrile containing copolymer, the change in the composition becomes serious in the respective reactors. In proportion to this, the amount of styrene monomers to compensate is increased in which case the transparency is deteriorated when mixing the monomers is insufficient.
Accordingly, an object of the present invention is to provide a novel method for preparing acrylonitrile-styrene copolymer excellent in color, transparency and mechanical property by maintaining the conditions of polymerization uniform in the preparation process.
The acrylonitrile-styrene copolymer prepared by the present invention method is excellent in mechanical property such as strength, resistance to thermal discoloration, and contains less gel-like polymer, as a result of which injection molded articles made of the resin has an excellent external appearance.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for preparing acrylonitrile-styrene copolymer by a continuous bulk or solution polymerization includes the steps of: (a) separately feeding 70 to 90 part by volume and 10 to 30 part by volume based on the total feed monomer into the first and second reactors from a raw material monomer storing section via two feeding sections, the raw material monomer consisting of 50 to 90 part by weight of styrene, 10 to 50 part by weight of acrylonitrile, 0 to 30 part by weight of polymerization solvent, and 0 to 0.5 part by weight of chain transfer agent, wherein an output is issued from the second reactor as much as the input at the same time, (b) polymerizing the mixture of the raw material monomers in each reactor for 1.5 to 2.5 hours at a reaction temperature of 130 to 160xc2x0 C. and under the pressure of 1.5 to 4 kg/cm2 G to have conversion ratio of 80%; and (c) recycling part of the reaction solution from the first reactor to a separate mixer, mixing discharged solution from the second reactor and part of monomers directly supplied from one feeding section, and recycling the mixture to the first reactor.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.